Fouling of equipment in fluid catalytic cracking (FCC) units can significantly affect unit operation by reducing the necessary transfer of heat in heat exchangers, by restricting unit throughput due to increased pressure drop and, in general, by reducing the overall operating efficiency of the production unit.
A loss in heat transfer can result in increased fuel costs to operate the unit or may affect product separation when the lost heat cannot be replaced by other means. The physical restriction of flow can cause production limitations due to increased pressure drop in the system. Pluggage in the separation towers can also restrict necessary separation efficiencies and subsequent product separation. The overall unit performance can be adversely affected, even when the flexability of unit operations exists to compensate for the effects of fouling.
FCC unit feedstocks are generally the heavier fractions from the upstream processing units. In those heavier gas oils, resids and other feeds, non-volatile, inorganic fouling materials tend to concentrate. As the fluids flow through the system, the individual smaller particles of the contaminants can agglomerate and form larger particles. Catalyst fines from the reaction process can be entrained in product streams and will contribute to inorganic foulants. Eventually, the settling velocity of the particles becomes higher than the local system velocity, and the particles settle out. They will settle first in the low-velocity portions of the system, such as the baffles, bends, and the trays of the tower. However, when other types of fouling, such as organic fouling, have already occurred, the rate of agglomeration can increase, thereby depositing the particles on other parts of the system.
The chemical composition of organic foulants is rarely identified completely. Organic fouling is caused by insoluble polymers which are sometimes degraded to coke. The polymers are usually formed by reactions of unsaturated hydrocarbons, although any hydrocarbon can polymerize. Generally, olefins tend to polymerize more readily than aromatics, which in turn polymerize more readily than paraffins. Trace organic materials containing hetero atoms such as nitrogen, oxygen and sulfur also contribute to polymerization.
Polymers can be formed by free radical chain reactions. These reactions, shown below, consist of three phases: an initiation phase, a propagation phase and a termination phase. Chain initiation reactions (1 a), (1 b), and (1 c) give rise to free radicals, represented by R. (The symbol R. can be any hydrocarbon).
Such chain reactions can be initiated by (1 a) heating a reactive hydrocarbon (e.g. olefin) to produce free radicals and (1 b), (1 c) the production of free radicals from an unstable hydrocarbon material via metal ions.
During chain propagation, additional free radicals are formed and the hydrocarbon molecules (R) grow larger and larger (see Reaction 2 a).
Through the termination phase free radical reactions are destroyed into nonradical products (3a, 3b, 3c). If free radicals are not destroyed, continued radical transfer leads to the formation of unwanted polymers.
As polymers form, more polymers begin to adhere to the heat transfer surfaces. This adherence results in dehydrogenation of the hydrocarbon and eventually the polymer is converted to coke.